MC68HC908GR16 Datasheet, PDF (80/310 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
4.8.3 Choosing a Filter
As described in 4.8.2 Parametric Influences on Reaction Time, the external filter
network is critical to the stability and reaction time of the PLL. The PLL is also
dependent on reference frequency and supply voltage.
Figure 4-10 shows two types of filter circuits. In low-cost applications, where
stability and reaction time of the PLL are not critical, the three component filter
network of Figure 4-10 (B) can be replaced by a single capacitor, C_F, shown in
Figure 4-10 (A). Refer to Table 4-4 for recommended filter components at various
reference frequencies. For reference frequencies between the values listed in the
table, extrapolate to the nearest common capacitor value. In general, a slightly
larger capacitor provides more stability at the expense of increased lock time.
CGMXFC
CGMXFC
C_F
R_F1
C_F2
C_F1
V_SSA
V_SSA
(A)
(B)
Figure 4-10. PLL Filter
Table 4-4. Example Filter Component Values
f_RCLK
32 kHz
40 kHz
50 kHz
60 kHz
70 kHz
80 kHz
90 kHz
100 kHz
C_F1
0.15 µF
0.12 µF
0.10 µF
82 nF
68 nF
56 nF
56 nF
47 nF
C_F2
R_F1
15 nF
2K
12 nF
2K
10 nF
2K
8.2 nF
2K
6.8 nF
2K
5.6 nF
2K
5.6 nF
2K
4.7 nF
2K
C_F
0.22 µF
0.18 µF
0.18 µF
0.12 µF
0.12 µF
0.1 µF
0.1 µF
82 nF
Data Sheet
80
Clock Generator Module (CGM)
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MC68HC908GR16 — Rev. 1.0
MOTOROLA